"The majority [of the injuries] were from contusions or internal injuries, blows," Hernández said. But of the first 45 dead that were laid out, he added, "they didn't have any evidence of broken bones or visible fractures. They were asphyxiated among themselves."
Survivors recounted from their hospital beds how their location inside the trailer determined who lived and who died.
Those unlucky enough to be riding jammed against the fragile walls of the freight container almost certainly died, survivors said. Those in the middle of the packed group survived, cushioned by their fellow migrants as the container flipped onto the road.
"The ones who died were the ones who were up against the walls of the trailer," said one young migrant from Guatemala being treated for a broken arm. "Thank God, we were in the middle. But the ones on the sides, they died."
The migrant, who did not want to give his name because he did not have proper documents in Mexico, described a gruesome scene of screaming and blood in the moments after the truck crashed into the base of the steel pedestrian bridge Thursday. He estimated about 250 migrants were on board.
The living had to extricate themselves from the tangled pile of dead and dying bodies.
"They fell on top of me, there were like two or three fellow migrants on top of me," the youth said.
Then came the grim task of trying to pull out the wounded.
"When I got out, another fellow migrant was shouting," the survivor said. "He was shouting to me, I pulled at him and got him out and put him on the side of the road, but he died."
The youth said the truck's driver, whose whereabouts was unknown, had entered a sharp curve at a high rate of speed and lost control on a road leading to Tuxtla Gutierrez, capital of Mexico's Chiapas state.
While the Mexican government is trying to appease the United States by stopping caravans of walking migrants and allowing the reinstatement of the "Remain in Mexico" policy, it has been unable to staunch the flood of migrants stuffed by the hundreds into trucks operated by smugglers who charge thousands of dollars to take them to the US border.
Guatemalan Foreign Minister Pedro Brolo called Friday for tougher sentences for immigrant trafficking.
"We have to toughen the punishment for those who profit off our people," Brolo said.
Survivors said most of those crammed into the trailer were from Guatemala and Honduras. Authorities said migrants from Ecuador, the Dominican Republic and Mexico were also aboard.
Those who spoke to survivors said they told of boarding the truck in Mexico, near the border with Guatemala, and of paying between $NZ3678 and $5149 to be taken to Mexico's central state of Puebla. Once there, they would presumably have contracted with another set of smugglers to take them to the US border.
Despite the cautionary tale, more migrants continued on the move toward the United States. On Thursday, after the accident, a group of almost 400 mainly Central American migrants walked down a highway leading to Mexico City, ignoring entreaties from immigration agents and National Guard troops to turn themselves in.
Mexican President Andrés Manuel López Obrador blamed poverty and lack of jobs that force people to migrate.
"We have been insisting that the causes that originate these unfortunate events must be addressed," said López Obrador, who has called on the United States to invest in development programmes for Central America. He said the US government is moving slowly on the issue.
Pope Francis, who visited Chiapas in 2015 and has made the plight of migrants a hallmark of his papacy, sent a telegram of condolences Friday to the archbishop of Tuxtla Gutierrez, offering prayers for the dead and their families, and for the injured.
